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Rubberized spray to seal rigid foam seams?

egentler | Posted in General Questions on


I’ve installed rigid foam on the top four feet of the interior walls of my basement. I plan to tape the vertical seams (and use cans of spray foam in some other places), but on the top and bottoms where the rigid foam meets the concrete, I’m looking for an easier way to seal it. Particularly where the panels meet the sill plate, caulking and cans of spray foam both seem like less than ideal options. I was thinking about buying some rubberized undercoating spray (ala Flex Seal) to seal some of the more difficult seams where the panels meet the concrete. Any thoughts? Will rubberized spray (or a version I could brush on) provide a legitimate moisture barrier to prevent mold? Is there any reason not to do this?


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  1. wjrobinson | | #1

    No to flex seal. There are commercial elastomerics. Martin?

  2. GBA Editor
    Martin Holladay | | #2

    The first question is why you are only insulating the upper 4 feet of your basement walls instead of insulating the entire wall down to the slab.

    Concrete is a conductor, so you have a big thermal bridge that you haven't addressed. With your approach, heat will flow through the concrete and out of your house; the insulation on the upper half of your wall will simply be bypassed. That's why you want to insulate the wall all the way down to the slab.

    For more information on this issue, see How to Insulate a Basement Wall.

    If you have a gap between the rigid foam panels and your sill plate, you should seal the gap with canned spray foam. If the gap is wide, you can first install a narrow piece of rigid foam, and then seal the cracks that remain with caulk or canned spray foam.

  3. egentler | | #3

    Thanks for your reply, Martin. I understand the wisdom of doing the whole wall. I did the foam on the upper half of the walls because that's the exposed part and I couldn't afford to do it all. I will likely finish the job at some point. The foam certainly has not been entirely "bypassed," though. While there is obviously some transfer through the concrete, the basement is much warmer for that four feet of foam being there.

    I understand that canned foam would be traditional, but it would be far easier to seal those seams with some other kind of sealer that could be brushed or sprayed on. What's wrong with rubberized undercoating or some kind of roof sealant? I'm not worried about insulation value at the seams--just looking for a moisture barrier. Spray foam seems like a tedious, expensive, and messy way to achieve that.


  4. ntisdell | | #4

    I like the idea of using a flex-seal type product - however this area is going to be one of your coldest spots in the winter and the spray rubber will provide little/no insulation value but will reduce the absorption/moisture buffer properties of concrete. Thus allowing possible condensation/frost build-up on a cold non-porus surface. Pooling.... Eventually mold possibly.

    aerosol-rubber in a basement doesn't sound very clean either.... lol

    I do really wish Dow or something would come out with more of a 'sprayfoam' type product - or a latex caulk spray that foams slightly... something you could laydown a 1/4"-1/8" foamy/caulky layer at a consumer level quantity and price. "Frothpacks" end up being high buck and beyond what most homeowners would use/buy.

  5. wjrobinson | | #5

    Nick, what you are asking for exists.

  6. ntisdell | | #6


    Where at? I haven't seen anything of that nature at HD, Menards, or Lowes...

    A small can of projectile spray foam in a can or similar 10-20oz for like $3-8?

    I have used Dap spray foam in a can (they label it more as a 'sealant') and various brands of GreatStuff like product.

  7. KeithH | | #7

    A couple options not mentioned above: If you have a spray foam gun (~<$40), you can use a product like Down Enerfoam which is also an adhesive. It might be too late for this but you could have used this product to adhere the foam to the concrete (assuming something else, like firring or framing, was providing backpressure). You won't find that enerfoam at HD or Lowes but your local lumber yard might sell it (~$20 for 30 oz, goes quite a long ways) or you could try an online vendor. I've ordered a foam gun and foam cans from a and been satisfied.

    Another option would be a high end european tape. I have not yet used these type of tapes on concrete but there is a primer designed to adhere them to concrete and they stick to wood and foam panels just fine. They are airtight (but vapor open) which seems appropriate for your application. I've been getting these tapes from but I imagine there are other similar products. They are spendy however.

    My last comment would be to use caution using rubberized products designed for the exterior of the building. If you (or your customer) has any sensitivity to VOCs or asphalt type odors, I'd test the product for odor or off-gassing first. I tried using a zip tape indoors to seal seams on some plywood indoors (shear panel). The asphalty odor was significant and I ended removing it all and replacing it with Rapidcell and Vana tapes (minimal to no odor).

  8. egentler | | #8

    Thanks guys,

    I guess I'll look into some of the tape options. Dow Weathermate, perhaps? Seems like people are down on rubberized spray. Beyond tape, I guess I'll do what I can with the old foam gun (and caulking) and then maybe get a froth pack for the hardest spots. Just seems like a lot of pain and money just to close off the seams to moisture. Maybe I can get the tape to work even at the concrete-insulation seams as you suggest, Keith. I'll give it a shot.


  9. fitchplate | | #9

    You ain't seen nothin' 'till you used Pro Clima tapes for air sealing and adhesion.

  10. mackstann | | #10

    I think I recall hearing about duct mastic being used on rigid foam seams.

  11. ntisdell | | #11

    Does the Enerfoam product shoot out a little bit as a wide spray (>1" spray) or more of a low expand line of foam (similar to DAP latex foam 'stuff') ?

  12. egentler | | #12

    Doesn't look like it. Looks a lot like the "Great Stuff" that we all use. I need something more like a froth pack, but cheaper.

  13. KeithH | | #13

    I think the Enerfoam is a little lower expansion than Great Stuff. I've used it to adhere and seal polyiso blocks (cut and cobble; worked great). If you get a good gun, you should be able to adjust the flow to lay down a controlled bead of either product however.

  14. UBuildIt_Indy | | #14

    Absolute best is Dockskin primer on the concrete, then Wigluv tape to adhere rigid to concrete. These are European products made by Siga. We use them to air seal houses. May be hard to find, but they are ideal for that application.

  15. bvillebound | | #15

    Dear Evan: We used the "Windows" version of Great Stuff to seal the seams, joints and holes in XPS foam board inside and outside. This version of Great Stuff remains flexible, which should eliminate cracks as the assembly expands and contracts. Great Stuff is also closed cell foam with an R value of roughly 3.7 per inch, and will not collect condensation.

    A couple of notes that may apply to your project:

    > Read up on fire blocking: You should install fireblocking at the top of your stud walls, e.g. 3/4" OSB to seal the connection between the top of the stud wall and the sill plate. A vertical firebreak is also required every 10' along a basement wall. Simply leave a gap in the foam board behind a stud, and pack the gap with rock wool. Always check with your inspector first; some inspectors only accept solid fireblocking, and some have unusual requirements for the 10' vertical fireblock. Here are a couple of helpful resources:

    > If you seal / fireblock holes for electric cables and plumbing, check the flammability of the product. I found that the "Fireblock" version of Great Stuff ignites at just 240 degrees F -- significantly lower than the ignition temperature for wood! We switched back to the non-flammable sealants made by 3M and DAP, which are sold at Home Depot and Lowes.

    I hope this is helpful.


  16. kevin_in_denver | | #16

    I agree that Dockskin and Wigluv is the best method.

    I'm also trying Henry's "Elastocaulk" because it's on the shelf at HD and much cheaper and faster for a 1/8" or less crack.

    I'm applying it with a 4" putty knife.

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